Technical Field
An aspect of the present invention relates to a glass substrate for mask blanks for use in various kinds of lithography, and to its production method.
The glass substrate for mask blanks in an aspect of the present invention is favorable for a glass substrate for mask blanks to be used in lithography using EUV (extreme ultra violet) light (hereinafter abbreviated as “EUVL”) (hereinafter this is abbreviated as “glass substrate for EUVL mask blank”).
The glass substrate for mask blanks in an aspect of the present invention is also favorable for a glass substrate for mask blanks for use in lithography using an already-existing transmission optical system, for example, for a glass substrate for mask blanks for lithography using an ArF excimer laser or a KrF excimer laser.
Background Art
With the recent tendency toward high-density and high-precision ultra-LSI devices, the specifications required for the surface of the glass substrate for mask blanks for use in various kinds of lithography are becoming severer year by year. In particular, with the wavelength of the light from the exposing source being shorter, the requirements for the profile accuracy (flatness) of the substrate surface and for the absence of the defects (particles, scratches, pits, etc.) in the surface are becoming severer, and a glass substrate having an extremely high degree of flatness and having few microdefects is desired.
For example, in a case of immersion lithography using an ArF excimer laser as the light from an exposing source, the necessary flatness of the glass substrate surface for mask blanks is 100 nm or less and the necessary defect size in the glass substrate is 70 nm or less; and further in a case of a glass substrate for EUVL mask blanks, the necessary flatness of the glass substrate surface is 30 nm or less as the PV value, and the necessary defect size is 50 nm or less.
To attain the above-mentioned high-level flatness, local processing of planarizing the surface of a glass substrate for mask blanks while locally controlling the processing level in accordance with the local protrusions in the substrate is employed. For the local processing method, plasma etching, gas cluster ion beam etching, a local processing tool using a magnetic fluid or a local processing tool using a rotary small-size processing tool is preferably used (see Patent Documents 1 and 2).
However, in the case where such a local processing tool is used for processing the surface of a glass substrate for mask blanks, scanning is necessary to be conducted by moving the local processing tool on the substrate thereof for processing the entire surface of the glass substrate for mask blanks. The scanning pitch interval with the above-mentioned local processing tool is typically from 0.1 to 1 mm, and in the part processed with the local processing tool, there may remain a periodical processing trace corresponding to the scanning pitch interval. For removing the processing trace, the entire surface of the glass substrate for mask blanks must be subjected to final polishing using a polishing pad and a polishing slurry.
On the other hand, in general, the circumference part of the surface of a glass substrate for mask blanks is provided with a surface to be chamfered for the reason of preventing cracking or chipping.
The above-mentioned requirements for flatness and defect size relate to the main surface excepting the circumference part provided with a surface to be chamfered in the surface of a glass substrate for mask blanks.
Patent Document 1: JP-A-2011-207757
Patent Document 2: Japanese Patent No. 5402391